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Trinity division, indicating that they are thinner and more arenaceous along the western border region, where they are best exposed, and that they thicken and become calcareous to the east, being buried in the region of their greatest development beneath the Black and Grand prairies, where a knowledge of their nature can be obtained only by careful interpretation of the artesian drillings. From the western border of the Grand Prairie to an indefinite distance beyond the eastern margin of the Black Prairie the embedded portion extends fully 40,000 square miles.

As a result of these variations of thickness and composition the rocks of the Trinity division along the eastern portion of the general area of exposure within the incised valleys of the Edwards Cut Plain present the aspect of several well-defined and mappable lithologic units of various kinds of rock, which so coalesce along the western border exposures into a general basement formation of sands that they are there usually inseparable.

As a whole these strata of the Trinity division may be subdivided into the following conspicuous formations and beds:

The name Antlers sands will be applied to the equivalents of all these formations as they coalesce along the western border region north of Parker County.

The three subdivisions are nowhere exposed in complete series. The Travis Peak deposits are exposed . in a small and irregular area in the valley of the Colorado in Blanco, Travis, and Burnet counties, between the mouths of Cypress Creek, Travis County, and Sycamore Creek, Burnet County. As elsewhere described in detail, they consist of coarse sands and conglomerates, beds of silicified shell breccia, and clays, probably the attenuated western representative of thicker limestone beds embedded eastward, above which is another group of sands and coarse grit, marked in places by ferruginous colors. In the author's opinion they represent the oldest of the Cretaceous formations exposed at the surface in the Texas section. They are possibly